1. Field of the Invention
The present invention relates generally to blow out preventers and more specifically to apparatus for equalizing pressures above and below the closure members or rams in a blow out preventer (B.O.P.) prior to opening the rams. After such equalization occurs, the rams can move more easily to their open positions because there is no pressure differential urging them transverse to their path of movement. This allows a smaller operator to be used for opening the rams. The ram seals are subject to less friction and enjoy a longer life. Such equalization prevents damage to the rams or blow out preventer cavity which may occur if the rams are somewhat cocked due to the pressure differential at the time they are forced open.
2. Description of the Prior Art
A number of prior patents describe schemes for attempting to ease the opening of blowout preventers by equalizing pressures in two different zones, typically using a bypass passageway which extends through at least one of the rams. However, such pressure equalization mechanisms have frequently been prone to intermittent leakage, are often mechanically complicated, and sometimes do not equalize at the optimum time or between optimum zones.
Some prior devices, such as that shown in U.S. Pat. No. 4,638,972 to Jones et al and that shown in U.S. Pat. No. 3,036,807 to Lucky, do not equalize the pressures in the well bore above and below the rams, but rather, equalize the pressure above the rams with that behind or laterally outwardly of the rams. This does reduce the pressure which must be overcome to open the rams, but does not eliminate the aforementioned problem of a higher pressure downhole urging the rams upwardly, i.e. transverse to their path of movement. Thus, these systems do not reduce the amount of force necessary to open the rams as much as they might, and they also fail to eliminate problems with wear of the rams, their seals, and the surfaces of their housing cavities.
The Jones et al system, and at least some of the embodiments of the Lucky systems, also suffer disadvantages in that the valve which opens and closes the bypass passageway seats and seals in an axial direction (with respect to its direction of motion). Thus, a slight flaw in the valve or valve seat, or their alignment when seated, can easily cause leakage. Likewise, a little debris on the valve element or valve seat (a not unlikely occurrence when well fluid must, of necessity, enter the bypass passageway) can interfere with proper seating of such an axially oriented valve, and likewise cause leakage. Furthermore, the slightest axial movement can unseat the valve.
Other prior systems do equalize the pressures above and below the rams prior to opening, but virtually all of these likewise utilize axially oriented valves, which suffer from all of the last-mentioned disadvantages. Examples are shown in U.S. Pat. No. 4,655,431 to Helfer.
One characteristic of B.O.P.'s that sometimes prevents a satisfactory seal in a pressure equalization mechanism is relative movement between the sealing mechanism and the ram. U.S. Pat. No. 4,943,031 to D.W. Van Winkle shows a sealing mechanism intended to adjust to such relative movement of internal B.O.P. components by using a spring-loaded plunger to centralize a ball valve element on a seat. The Van Winkle mechanism has a number of problems besides the fact that his valve is axially oriented. Several moving parts are necessary. Also, the B.O.P. may not seal effectively if the pressure above the rams is higher than the pressure below because the higher pressure above the rams will have a tendency to unseat the spring-loaded ball. Such a condition may occur when ensuring proper installation of any pressure control equipment that may be added above the B.O.P.'s. Standard practice for many companies requires closing the B.O.P.'s and applying a high pressure above the B.O.P.'s to test such additional equipment, in other words a reverse differential pressure. With the Van Winkle spring-loaded ball valve, it may appear that the additional equipment being tested is leaking when, in fact, the ball valve in the Van Winkle mechanism may have become slightly unseated due to the pressure reversal. Finding such a leak may consume substantial time.
Similar problems may occur in the aforementioned U.S. Pat. No. 4,638,972 to Jones et al (in addition to those problems already mentioned) since, in that device, a pressure differential acting on the seat is relied upon to aid in making a tight seal. If the pressure differential is small or reversed, sealing problems may develop.
Aforementioned U.S. Pat. No. 3,036,807 to Lucky does appear to disclose at least one embodiment utilizing a non-axially-oriented seal. However, as previously mentioned, Lucky does not fulfill the basic requirement of equalizing the pressures above and below the rams, in the borehole, prior to opening of the rams. Also, Lucky's construction makes it difficult to remove and install the rams for replacement or repair independently of the actuators.
The mechanisms described above fail to provide a highly reliable and simply constructed seal which, when released, will equalize the pressures above and below the rams prior to opening. Those skilled in the art have long sought and will appreciate the novel features of the present invention which solves these problems.